One challenge in the development of antennas for mobile handheld devices, such as cellular devices, is the balance between the antenna size and its performance. On one hand, users have come to expect smaller and relatively stylish devices with no visible antenna structure, which imposes restrictions on the device form factor and thus the available antenna size. On the other hand, users expect devices with an antenna that efficiently supports the various wireless communications standards. Yet, this requires that the antenna has a reasonable size to achieve requisite performance often over multiple operating frequency bands. See, e.g., Geyi, “Physical Limitations of Antenna,” IEEE Transactions on Antennas and Propagation, vol. AP-51, pages 2116-2123, 2003.
Planar inverted-F antennas (PIFAs) are commonly used for handheld devices. However, PIFAs typically have relatively narrow bandwidths. To overcome this shortcoming, various techniques are sometimes used to increase the effective bandwidth of PIFAs, such as using parasitic elements, additional shorting pins, etc. Yet, such structures can unduly complicate the antenna structure and increase its cost. See, e.g., U.S. Pat. No. 7,023,387; Liu et al., “Dual-Frequency Planar Inverted-F Antenna,” IEEE Transactions on Antennas and Propagation, vol. 45, no. 9, pages 1451-1457, October 1997; Rowell et al., “A Compact PIFA Suitable for Dual-Frequency 900/1800-Mhz Operation,” IEEE Transactions on Antennas and Propagation, vol. 46, pages 586598, April 1998; Guo et al., “Miniature Built-In Quad-Band Antennas for Mobile Handsets,” IEEE Antennas Wireless Propagation. Letters, vol. 2, pages 30-32, 2003.
Another form of antenna, i.e., the monopole antenna, typically has a relatively wider bandwidth as compared with that of a PIFA. However, a significant drawback of such monopole antennas is that they typically require more surface area (i.e., they are larger) than a comparable PIFA. Another drawback of monopole antennas is that, due in part to the size constraints, they are typically implemented as external antennas, whereas a PIFA is easier to implement as an internal antenna.
Even so, another advantage that a 2D monopole antenna has over the PIFA, in addition to its wideband response, it has a low profile, is simpler to design, and less expensive to fabricate.
One exemplary monopole antenna arrangement is set forth in U.S. Pat. No. 6,054,955 to Schlegel, Jr., et al. The antenna arrangement is for use in the housing of a portable communications device, such as a laptop. The antenna arrangement includes a pair of spaced folded monopole antennas in 2D. Each antenna includes a first printed circuit board having a conducting surface that forms a ground plane. Mounted on the first circuit board is a second printed circuit board having a right-angled strip of conducting material, which forms a folded monopole radiating element. The folding of the monopole reduces its height, to thereby enable it to fit into small casings and the like. To compensate for the effects of the folded monopole on the electrical match, frequency bandwidth and electromagnetic fields, a shunt inductance is introduced between the monopole and the ground plane. The antennas are mounted within cavities that can be lined or coated with metallic material, to improve the radiation patterns of the antennas and isolate them from the electronic components of the communications system.
Despite the existence of such antenna arrangements, further advancements in monopole antenna structures for mobile wireless communications devices may be desirable in some applications.